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The fallacy of the single cause, also known as causal oversimplification, is a fallacy of questionable cause that occurs when it is assumed that there is a single, simple cause of an outcome when in reality it may have been caused by a number of only jointly sufficient causes.

Often after a tragedy it is asked, "What was the cause of this?" Such language implies that there is one cause, when instead there were probably a large number of contributing factors. However, having produced a list of several contributing factors, it may be worthwhile to look for the strongest of the factors, or a single cause underlying several of them. A need for simplification may be perceived in order to make the explanation of the tragedy operational, so that responsible authorities can be seen to have taken action.

For instance, after a school shooting, editorialists debate whether it was caused by the shooter's parents, violence in media, stress on students, or the accessibility of guns. In fact, many different causes—including some of those—may all have necessarily contributed.

Causal oversimplification is a specific kind of false dilemma where conjoint possibilities are ignored. In other words, the possible causes are assumed to be "A or B or C" when "A and B and C" or "A and B and not C" (etc.) are not taken into consideration.

A notable scientific example of what can happen when this kind of fallacy is identified and resolved is the development in economics of the Coase theorem.